The present invention relates to ventilated filters for cigarettes, and provides such a filter comprising longitudinally aligned core components a first of which is air-permeable and of relatively high pressure drop longitudinally of the filter but substantially air-impermeable radially of the filter and a second of which is air-permeable and of relatively low pressure drop longitudinally of the filter, and a common full or partial wrap which extends along and around or partially around said core components and provides for ventilation of the filter at a region longitudinally spaced from the first core component. The first and second core components may abut or be longitudinally spaced, and the ventilation may, for example, be radially into a radially air-permeable second component and/or into a space between first and second core components. The wrap may be a complete wrap of porous or perforate material of substantially uniform air-permeability along its length. It may instead be a partial wrap comprising one or more strips, the or each of which extends only partially around the filter core to leave between longitudinal strip edges gaps which extend longitudinally of the filter and into which free ventilation can occur.
The first core component is preferably radially impermeable over the whole of its cross section. It is preferably a unitary body, except for any outer wrapper which may form part of it. It is preferably substantially uniform over the whole of its cross section. It preferably occupies fully and uninterruptedly the whole of the cross section (other than that occupied by the common wrap or partial wrap) of the filter. The first core component thus suitably comprises a unitary substantially uniform plug of circular cross section which may include its own wrap.
In a filter cigarette, the filter according to the invention will normally be incorporated with the high pressure drop core component towards the tobacco rod and with ventilation into the filter downstream of the high pressure drop component towards the buccal end. The filter will normally be incorporated in a filter cigarette by means of a ventilating tipping overwrap, which will usually have ventilating perforations disposed downstream of the high pressure drop core component. The ventilating tipping overwrap may be the previously mentioned common wrap for the first and second core components, but more usually these core components will be preformed into a sub-assembly with the common wrap, and this sub-assembly subsequently incorporated in a filter cigarette by means of the ventilating tipping overwrap. The sub-assembly is preferably produced continuously, with the continuous rod being cut into finite lengths as it is produced; these finite lengths will usually be a multiple of the eventual individual filter lengths; in the production of filter cigarettes, a double length rod will usually be aligned longitudinally between two tobacco rods, joined thereto by a double-length tipping overwrap, and the assembly then cut centrally to form two filter cigarettes.
Various structures and materials are possible for the high pressure drop core component. It is currently preferred to employ plastics film, longitudinally corrugated, substantially without fibrillation so as to remain air-impermeable, and gathered laterally to form a filter plug which is permeable longitudinally, along the corrugations, but substantially impermeable radially. The gathered corrugated film may be bonded to itself to provide a self-supporting and dimensionally stable rod or plug, but more usually it will be retained in rod form by means of a wrapper. The plastics film is suitably of polyethylene.
The low pressure drop core component may be of conventional form, e.g., of gathered cellulose acetate filamentary tow or gathered creped porous paper. It is preferably a unitary body. It may be radially air-permeable. It is preferably substantially uniform over the whole of its cross section. It may occupy fully and uninterruptedly the whole of the cross section (other than that occupied by any wrap) of the filter. The second core component thus suitably comprises a unitary substantially uniform plug of circular cross section.
Each of the first and second core components, and the filter as a whole, is preferably of low inherent mechanical retention. In particular, in filters and filter cigarettes according to the invention, the percentage air-dilution via the filter is preferably greater than the percentage "non-ventilated" or "enclosed" tar retention of the filter, i.e., that measured for an equivalent filter or cigarette with air dilution via the filter prevented. The percentage air dilution, as referred to herein, is the percentage by volume of ventilating air added via the filter in the total mixture delivered by the filter; thus 50% ventilation or air dilution means that in each puff there is a 50/50 volume ratio of added air to original smoke, and 40% air dilution indicates a 40/60 ratio, and so on.
The invention relies on the relative properties of the first and second core components combined with the extent and location of ventilation) rather than on the absolute values of the properties of either. However, while this is by no means essential for a filter performing according to the invention, the first core component will generally have an enclosed pressure drop (i.e., that measured with ventilation prevented) of over 50 mm water gauge (Wg) and the second core component an enclosed pressure drop of less than 50 mm Wg. For a given type (composition, structure, packing density, etc.) of first core component the enclosed pressure drop will be proportional to length, and the same applies to the second core component; the enclosed pressure drops of the two components will be substantially additive, so that a desired enclosed pressure drop for the filter is readily achieved. For a second core component in the form of a conventional filter plug (e.g., gathered cellulose acetate tow) the variation of enclosed tar retention with enclosed pressure drop is usually substantially linear, with a ratio greater than unity, and an enclosed tar retention of less than 30% will generally be preferred.
For the first core component, there is preferably substantially less variation of enclosed tar retention with enclosed pressure drop, and an enclosed tar retention of less than 20% will usually be preferred. The enclosed tar retention of the filter will preferably be under 40%.
The filter according to the invention can achieve the usually irreconcilable objectives of on the one hand permitting a very high degree of air-dilution (e.g., 50% or 60% or more) to give good reduction of CO without, on the other hand, reducing the taste and pressure drop of the filter cigarette to unacceptably low levels. Various prior filter structures have aimed at this performance, but the filter of the present invention can achieve it to an improved degree by use of a very high pressure drop, low retention upstream core component in combination with a low pressure drop, low retention downstream core component, with high air dilution downstream of the high pressure drop component; the improved performance is obtainable with a filter which is of essentially uniform cross section, without implants or capillary tubes etc. which have been proposed for the purpose in the past and which can become blocked to impair the performance; the whole cross section of the filter can be used, and the filter can be of conventional appearance and would present an even end staining during smoking; furthermore, in view of the very high pressure drop which can be provided by the first (upstream) core component, it is relatively easy to obtain the high air dilution levels desired without having to use exceptionally porous tipping, relatively small ventilation perforations being adequate; the combination of small ventilation holes and high upstream pressure drop can prevent or reduce the escape of smoke through the ventilation holes between puffs, the latter being an undesirable phenomenon frequently encountered with prior ventilated filters.
The invention is further illustrated by the following practical data and examples to be taken in conjunction with the accompanying drawings.